1. Field of the Invention
The present invention relates in general to a fluid-filled elastic mount for damping or isolating vibrations based on flows of a non-compressible fluid contained therein. More particularly, the present invention is concerned with such a fluid-filled elastic mount which is simple in construction, and is capable of exhibiting different vibration damping or isolating characteristics depending upon the type of the vibrations applied thereto.
2. Discussion of the Prior Art
An elastic mount is known as a vibration damping device interposed between two members of a vibration system, for flexibly connecting these two members. As a type of this elastic mount, there is known a so-called fluid-filled elastic mount as disclosed in laid-open Publication No. 55-107142 of unexamined Japanese Patent Application. The elastic mount disclosed in this publication includes a first and a second support structure which are spaced apart from each other in a load-receiving direction and are elastically connected to each other by an elastic body interposed therebetween. The elastic mount further includes a partition member supported by the second support structure so as to extend in a direction substantially perpendicular to the load-receiving direction, and thereby cooperate with the elastic body to at least partially define a pressure-receiving chamber on the side of the first support structure. On the other side of the partition member on the side of the second support structure, there is provided a variable- volume equilibrium chamber which is partially defined by a flexible diaphragm. These pressure-receiving and equilibrium chambers are filled with a non-compressible fluid, and communicates with each other by an orifice passage. A pressure in the pressure-receiving chamber changes due to elastic deformation of the elastic body upon application of vibrations in the load-receiving direction. The fluid-filled elastic mount damps or isolates the input vibrations based on the resonance of the fluid mass flowing through the orifice passage more effectively than an elastic mount which relies on only the elastic property of the elastic body for damping the input vibrations. For this reason, the elastic mount of this type is widely used for motor vehicles, for example, as a mount for mounting the engine on the vehicle body.
Generally, the fluid-filled elastic mount, when used as a vehicle engine mount, is required to exhibit a high degree of damping characteristic with respect to low-frequency vibrations, such as engine shake and bounce, and exhibit a sufficiently low dynamic spring constant with respect to medium-frequency vibrations such as engine idling vibrations, and high-frequency vibrations such as booming noise. However, an improvement in the vibration isolating function based on the resonance of the fluid mass flowing through the orifice passage can be provided with respect to only the vibrations whose frequencies are in the neighborhood of the frequency to which the orifice passage is tuned. Where the orifice passage is tuned to effectively damp the low-frequency vibrations based on the resonance of the fluid mass flowing through the orifice passage, the orifice passage operates as if the passage was substantially closed when the mount receives the vibrations whose frequencies are higher than the tuned frequency of the orifice passage. As a result, the elastic mount suffers from an undesirably increased dynamic spring constant, and from a considerable deterioration of the damping or isolating function with respect to the medium- to high-frequency vibrations.
In view of the above drawback of the known fluid-filled elastic mount, a fluid-filled elastic mount with a pressure absorbing system is proposed as disclosed in laid-open Publication No. 57-9340 of unexamined Japanese Patent Application. In this fluid-filled elastic mount, the partition member disposed between the pressure-receiving and equilibrium chambers is provided with a movable member which can be displaced by a predetermined distance in the direction of opposition of the two chambers (i.e., the load-receiving direction). When the elastic mount receives high-frequency vibrations, this arrangement serves to avoid an unfavorable increase in the dynamic spring constant of the elastic mount resulting from the substantially closed orifice passage, since a pressure change which occurs in the pressure-receiving chamber is absorbed by the displacement of the movable member.
The pressure absorbing system used in the above-described elastic mount is effective to isolate the high-frequency, small-amplitude vibrations. However, when the amount of displacement of the movable member is set to be a comparatively large value, in an attempt to provide an effectively lowered dynamic spring constant with respect to the medium-frequency vibrations having relatively large amplitudes, the movable member may be displaced to an extent to absorb a pressure change in the pressure-receiving chamber which occurs upon application of the low-frequency vibrations as well as the medium-frequency vibrations. In this case, a sufficient amount of the fluid is unlikely to flow through the orifice passage upon application of the low-frequency vibrations, resulting in deterioration in the vibration damping characteristic of the mount for the low-frequency vibrations. Thus, the proposed elastic mount is incapable of providing a sufficient degree of isolating characteristic with respect to the medium-frequency vibrations such as engine idling vibrations.
To solve the problem encountered in the proposed fluid-filled elastic mount, it is proposed to provide two mutually independent orifice passages, one of which is tuned to provide a sufficiently reduced dynamic spring constant so as to isolate the medium- to high-frequency vibrations, and the other of which is tuned to provide an excellent damping effect for the low-frequency vibrations.
The above-indicated one of the two differently tuned orifice passages is tuned so that the resonance frequency of the fluid mass flowing therethrough is higher than that of the fluid mass flowing through the other orifice passage, and so that the flow resistance of the fluid in the one orifice passage is smaller than that of the fluid in the other orifice passage. Therefore, the fluid filling the elastic mount is unlikely to flow through the other orifice passage having a relatively high flow resistance of the fluid. Consequently, it is rather difficult to ensure a sufficient amount of the fluid flowing through the other orifice passage so as to effectively damp the low-frequency vibrations.
In view of the above drawback of the proposed fluid-filled elastic mount, the applicant of the present application proposed another fluid-filled elastic mount as disclosed in laid-open Publication No. 60-220239 of unexamined Japanese Patent Application, which includes a control valve for selectively opening and closing the above-indicated one orifice passage tuned to the higher resonance frequency, and an actuator for operating the control valve. The control valve is operated by the actuator, so that the above-indicated one orifice passage is automatically opened and closed to permit the elastic mount to exhibit different vibration isolating or damping characteristics, based on the fluid flows through the selected one of the two orifice passages, depending upon the type of the input vibration.
In the thus constructed fluid-filled elastic mount, however, the provision of the control valve disposed in the relevant orifice passage along with the actuator disposed in the elastic mount results in increased structural complexity and a comparatively large size of the elastic mount. Accordingly, the cost of manufacture of the elastic mount is inevitably increased.